Abstract

Dolomitised Winnipegosis buildups of Middle Devonian (Givetian) age occur throughout the Elk Point Basin in southeastern Saskatchewan and North Dakota. In 1986, the first commercial reserves of hydrocarbons in the Winnipegosis Formation were proven by the Home Souris River 8A-22-2-9W2M Tableland well. Subsequent drilling in the Tableland pool resulted in estimated recoverable reserves of 250 000 m³ of oil.

Winnipegosis buildups in the Tableland area consist of five distinct facies. The lower two thirds of the buildups consist of peloids and codiacean algae which grew on the Lower Winnipegosis platform as a series of coalesced porous grainstone shoals. The windward flanks of the shoals were stabilised by early marine fibrous cements. The upper one third of the buildups consist of framebuilding organisms which exhibit a well-defined zonation. At the windward, northeast margin, digitate red algae (Solenopora) and stromatolitic “blue-green” algae formed a rigid, wave resistant margin. In other areas, Thamnopora, chaetetids and “blue green” algae occupied a more protected niche.

Diagenetic products which were formed during subaerial exposure of the buildups include calcrete crusts, pendant cements and vadose silt and mud. Later diagenetic products include blocky isopachous and blocky mosaic cements and syntaxial overgrowths, developed in the shallow burial environment. Limpid dolomite and anhydrite cements were precipitated in an intermediate to deep burial environment.

The reservoir at Tableland is heterogeneous and its distribution is the product of primary, depositional facies and subsequent diagenetic events. The best reservoirs exist in the grainstone shoals of the buildup interior. On the windward flanks and on the buildup crest, interparticle, fracture and cavernous porosity is occluded by marine cements and vadose silt and mud.